Collinear input and output couplers, each using rectangular guide to ridge guide to transmission line conversion, for traveling wave tube



April 26, 1966 w. A. SMITH, JR 3,248,601

COUPLERS, EACH USING RECTANGULAR COLLINEAR INPUT AND OUTPUT GUIDE TO RIDGE GUIDE TO TRANSMISSION LINE CONVERSION, FOR TRAVELING WAVE TUBE Filed. Oct. 50, 1957 2 Sheets-Sheet l April 26, 1966 w SMITH, JR 3,248,601

UT COUPLERS, EACH USING RECTANGULAR COLLINEAR INPUT AND OUTP GUIDE TO RIDGE GUIDE TO TRANSMISSION LINE CONVERSION, FOR TRAVELING WAVE TUBE Filed Oct. 30, 1957 2 Sheets-Sheet 2 jA/VENTOQ W/LL/AM 4 SMITH, JQ. 57 89W Q/VEY United States Patent 3,24%,dil1

CULLINEAR INPUT AND UUTPUT (ZQUPELERS, EACH USING RECTANGULAR GUIDE Til RIDGE GUIDE TO TRANMISSEON MINE CUN- VERSHQN, FUR TRAVELING WAVE TUBE William A. Smith, .lr., Milton, Mass, assiguor to Raytlieon Klompany, a corporation of Delaware Filed Get. 39, 1957, Ser. No. 694,693 9 Claims. (Cl. 31.5-39.3)

This invention relates to a radio-frequency coupling system for a microwave electron discharge device, and, more particularly, to a collinear waveguide coupler for providing a transmission path from a source of radiofrequency input energy to a traveling wave tube and a transmissionpath from the traveling wave tube to an output means. Y

As is well'knowmjtraveling wave tubes make use of the interaction between an electron beam projected along a slow-wave propagating structure and the electromagnetic field of the wave guided by said structure. The wave fields of periodic structures can be resolved into a series of so-called space harmonic waves which travel with diflierent phase velocities and electron beams can be made to interact with one of the components of the electromagnetic field by varying the electron velocity so that it is in substantial synchronism with the phase velocity of one of the space harmonic wave components. In the case of some of the space harmonics, known as forward waves, the phase and group velocity is in the same direction; on the other hand, backward waves are those in which the phase and group velocities are oppositely directed. Amplification and generation of oscillations may be obtained by interaction between an electron beam and forward waves or by interaction between an electron beam and backward waves.

In any of the traveling wave'tubes of the prior art, the input and output coupling members are mounted about the periphery of the tubes and protrude a considerable distance from the body of the tubes. These radially projecting members consume a great deal of space and such tubes are bulky and unattractive. The section of the tube occupied by these projecting coupling members cananot be utilized for effective interaction between the electron beam and the fields of radio-frequency energy traveling along the periodic, slow-wave propagating structure. In the usual tube designs, the input and output terminals are located close together; this arrangement, however, requires that conventional types of input and output transmission lines be tapered in order to be accommodated mechanically by the tube and so as not to perturb the electrical characteristics of the tube. If, however, the input and output transmission lines are tapered, electrical and mechanical design difiiculties are introduced. Where separate radial input and output connections are used, as in conventional tubes, two vacuum-tight brazes may have to be made relatively close together. In the device of the invention, however, only one vacuumtight seal is required, and the assembly techniques required for the tube assembly are greatly simplified.

When a cascade arrangement of traveling wave amplifier tubes is required, the above problems become correspondingly magnified. In such cascade arrangements using tube assemblies according to the invention, the coupling means between tubes may be arranged along a straight line. Furthermore, the main body of the traveling wave tube must support bulky couplers now in widespread use, thereby adding mechanical restrictions to the tube, often resulting in an increase in tube size and weight.

In accordance with the invention, a traveling wave tube coupling system is obtained which is mechanically simple, economical to fabricate and visually attractive and which overcomes the aforesaid disadvantages of prior coupling arrangements. In some cases, the body of the tube may be supported by the single length of waveguide, thereby reducing the mechanical limitations on tube design.

A run of rectangular waveguide is divided into two equal lengths by the insertion of a thin metallic partition with its plane substantially normal to the waveguide axis. This partition, when affixed to the walls of the waveguide, fills the cross section of the latter and thereby eliminates direct transfer of energy from one portion of the waveguide to the other. On both sides of the partition, pairs of ramps attached to the waveguide form sections of double-ridge waveguide. The outer ends of the doubleridge waveguide are tapered exponentially to provide dcsirable electrical properties, while the inner ends are abruptly terminated at a point close to the isolating partition. At this point parallel plane or two wire transmission lines having one conductor attached to each ramp proceed in a direction substantially normal to the waveguide axis and parallel to the partition through a single aperture in the side of the Waveguide into the tube. Inside the tube one pair of conductors attaches to the input terminals of the slow-wave propagating structure and the other attaches to the output terminals thereof. If so desired, the isolating partition may be extended to the body of the tube through the aperture in order to suppress coupling across the gap between input and output terminals of the slow-wave structure. The system, including the tube, is evacuated by means of windows transparent to radio-frequency energy; these windows are located at both ends of the rectangular waveguide.

Further objects and features of this invention will be understood more clearly and fully from the following detailed description of the invention with reference to the accompanying drawings, wherein:

FIG. 1 is a view, partly in section, of a collinear coupling arrangement for a traveling wave tube according to the invention;

FIG. 2 is a pictorial view of a complete tube assembly including the external magnet and the Waveguide coupler;

FIG. 3 is a view showing the connection of the input and output parallel wire lines to the slow-wave structure of the traveling wave tube;

FIG. 4 is a view showing further details of the device of FIG. 1;

FIGS. 5 and 6 are views illustrating construction de tails of the input and output parallel wire lines used with an interdigital type slow-wave structure;

FIG. 7 is a detail view illustrating the use of parallel plate transmission lines rather than parallel wire transmission lines between the ramp portions of the waveguide and the input terminals of the slow-Wave structure;

FIG. 8 is a detail view illustrating a means for impedance-matching the waveguide to the input terminals of the slow-wave structure by means of a change in spacing of the two wires of the transmission line;

FIG. 9 is a detail view illustrating means for impedance-matching the waveguide and the slow-wave structure of the tube by means of a quarter-wave transformer;

FIG. 10 is a detail view of a portion of the slow-wave structure indicating an alternative method of connection to the two wire transmission lines; and

FIG. 11 is a detail view showing a modification of the arrangement of FIG. 1 wherein the ramps may be replaced by reentrant portions of opposed waveguide walls.

Referring to the drawing, a traveling wave tube is indicated by the reference numeral 12 and includes a slow.- wave structure 14, a cathode assembly 16 (shown in FIG. 2) arranged substantially concentric with the slowwave structure parallel to the longitudinal axis of the tube, and a magnet assembly 18 (shown in FIG. 2). A typical slow-wave structure of the vane type is indicated in FIGS. 1 and 3, although the invention is limited only to a non-reentrant periodic slow-wave structure. An alternative type ofdelay structure is shown in FIGS. 5 and 6 and will be described subsequently. The cathode assembly has been omitted from FIGS. 1 and 3 for the sake of clarity but may be similar in construction either to that shown in a copending application, Serial No. 562,-

472 of Edward C. Dench and Albert D. La Rue, filed January 31, 1956, now Patent No. 2,888,649, or to that shown in US. Patent No. 2,681,427 of W. C. Brown et al., issued June 15, 1954, both of which are assigned to Raytheon Company of Lexington, Massachusetts.

, The aforesaid copending application discloses a cathode of limited size positioned adjacent a cylindrical auxiliary electrode or sole which forms one boundary of the interaction space, while the above-mentioned patent reveals a cathode having a more or less electron-emissive surface with the slow-wave structure. An electric field may be produced between the slow-wave structure and the sole, in the case of the tube described in the aforesaid application, or between the slow-wave structure and a continuous electron-emissive electrode such as referred to in the aforementioned patent.

The magnet assembly 18 produces a magnetic field transverse to the interaction space between the delay structure and the cathode. This magnet assembly 18 consists of a plurality of parts 18a, 18b, 18c, and 18d joined together by fastening means 33, as indicated in FIG. 2. It is possible, of course, to employ a magnet 18 made of one piece, although this is more difficult to fabricate.

The slow-wave structure 14 shown in FIGS. 1 and 3 includes a plurality of radially disposed vanes 21 projecting inwardly from a cylindrical envelope 22. A pair of end plates 23 and 24 are provided at opposite ends of the vanes 21 and form one boundary of the slow-wave structure. Although the vanes 21 are shown connected to the inner periphery of envelope 22, they need not be connected thereto either mechanically or electrically. A pair of straps 25 and 26 connect with alternate vanes 21, as shown clearly in FIG. 3. The assembly 14 is hermetically sealed'by means of end members 28 and 29.

An aperture 30 is provided in the envelope 22 of tube 12 through which two parallel wire lines 32 and 52 formed outside the tube envelope may be brought into the tube interior. The input parallel wire line 32 consists of two parallel electrical conductors 34 and 35, while output parallel wire line contains two parallel conductors 54 and 55.

A double ridge waveguide 36 includes a first or input section 37 and a second or output section 38 electrically isolated from the input section 37 by an electrically conductive partition or septum 39. The ends of the Waveguide sections 37 and 38 remote from the septum are provided with flanges 37 and 38 respectively, for connecting the waveguide with various input and output circuit components. Output waveguide section 38 includes a pair of juxtaposed ramps 40 and 41 which may be section 37 includes a pair of ramps 46 and 47 secured secured to opposite walls 43 and 44, respectively, of the Waveguide, as by screws 45. Similarly, input waveguide to opposite walls 48 and 49, respectively, of waveguide section 37. The ramps 40, 41, 46, and 47 are tapered exponentially at the outer ends, that is, the ends remote from septum 39, in order to provide proper impedance matching between the waveguide 36 and the parallel wire line 32. Since the impedance of the rectangular waveguide normally is in excess of the impedance of the parallel wire line, the spacing between ramps is progressively decreased as the parallel wire line is approached. The impedance matching is improved as the tapered portion of the ridge formed between ramps is made longer; however, a compromise in impedance matching must be made in order to reduce the length of the waveguide-traveling wave tube assembly. A single ridge waveguide is within the purview of this invention and can be obtained simply by eliminating one of the ramps of each pair. The double ridge waveguide, however, is more symmetrical and is better adapted to operations with the parallel conductor line and strapped slow-wave structure shown in FIGS. 1 and 3.

If the spacing between the ridges, that is the spacing between ramps, is too large, the distribution of energy outside the ramps will be too great and much of this energy will be refiectedfrom septum 39, resulting in a poor impedance match between waveguide 36 and the transmission lines. For a given spacing between ramps, the impedance is decreased as the breadth of the ramps is increased.

One conductor 34 of two wire transmission line 32 is connected to ramp 46 at the end thereof adjacent to septum 39, while one end of the other wire 35 of line 32 is connected to ramp 47 which faces ramp 46, as indicated in FIG. 1. The wires 34 and 35 extend substantially normal to the waveguide axis and parallel to septum 39 through an aperture 58 in the wall 57 of the waveguide and an aperture 30 -in the cylindrical portion 22 of the traveling wave tube 12. The aperture 58 in wall 57 of the waveguide is more or less coextensive with aperture 30. The other end of conductor 34 of transmission line 32 is connected to one end of strap ring 25 and the other end of conductor 35 of transmission line 32 is attached to the corresponding end of the strap ring 26.

The conductors 54 and 55 of output transmission line 52 interconnect the ramps 40 and 41 of the output section 38 of the waveguide 36 and one end of the strap rings 25 and 26, all respectively. The wires of each transmission line 32 and 52 should not be too close to the partition 39 lest the latter affect the field existing about the transmission line.

The isolating partition 39 may extend into the body of the tube 12, as indicated in FIG. 10, to minimize coupling between the input and output terminals of the slow-wave structure 14,

Energy from a driving source, not shown, may be supplied to the input waveguide 36 in the direction of the arrow in FIG. 1; this input energy passes through waveguide section 37, input transmission line 32, traveling wave tube 12, output-transmission line 52, waveguide section 38, and thence to a load (not shown), in the order named.

This invention may be used in a traveling wave oscillator, as well as in an amplifier; when the tube is used as an oscillator, one complete waveguide section and one parallel conductor transmission line obviously would be omitted.

As shown in FIGS. 5 and 6, the slow wave structure 14 may be an interdigital structure with a plurality of fingers 13 spaced from a back wall 15 which is substantially equivalent to the cylinder 22 of FIGS. 1 to 3. The two conductors 34 and 35 of transmission line 32 may be connnected to successive fingers at or adjacent to the input end of the slow-wave structure 14. Likewise, the two conductors 54 and 55 of transmission line 52 may be connected to adjacent fingers which are at different potentials at a given instant of time. It will be noted that either end of the slow wave structure 14 may be the input end; the other end of the structure 14 then would be the output end. Likewise, the portion 37 of the waveguide 36 may be the output end and the portion 38 of the waveguide the input end.

One of the conductors 35 and 55 of each of the transmission lines 32 and 52 may be connected directly to a vane 21, as shown in FIG. 10, rather than to one of the straps. The conductor of the transmission line is directly connected to a vane which is not connected to a strap to which the other conductor is connected. The vanes 21 of the slow-wave structure 14 may be recessed as shown in FIG. 10. The location of straps 25 and 26 relative to the vane tips facing the interaction space between structure 14 and cathode 16, will depend upon the desired electrical characteristics of the slow-wave structure.

In order to preserve a parallel relation of the conductors of the transmission lines over as great an extent as possible, the conductors in the arrangement of FIGS. 5 and 6 are bent near the point of connection to the fingers of the slow-wave structure 14. Although a limited cathode 16 is indicated in FIG. 6, together with a continuous electrode 19 which forms one boundary of interaction space 20, the cathode 16 may be omitted and electrode 19 coated with an electrically emissive material to form a continuous cathode.

The transmission lines shown in FIGS. 1' to 6 do not transform impedance; such lines are suitable in cases where the impedance at the junction of the waveguide and the transmission line and that at the junction of the transmission line and the traveling wave tube input impedance are substantially equal. In those cases, however, where the aforesaid impedances are quite different, it is necessary to provide an impedance match to avoid undesirable reflection of energy. This may be achieved, for example, by the methods illustrated in FIGS. 8 and 9.

As shown in FIG. 8, the spacing of the conductors 54 and '55 of transmission line 52 may be changed progressively. The arrangement of FIG. 8 represents a progressive increase in impedance along the line 52 for matching a waveguide of one impedance to a traveling wave tube of higher impedance. If the traveling wave tube impedance were lower than that of the waveguide, the spacing between conductors of line 52 would be smallest at the traveling wave tube end of the line. It should be noted that the same construction is applicable to transmission line 32, not shown in FIG. 8. This is likewise true of the arrangement of FIG. 9. This method of impedance transfer obviously is applicable to parallel plate lines, as well as to parallel wire lines.

Another method of impedance transfer is shown in FIG. 9 wherein a portion 52a of the transmission line 52 approximately any odd number of quarter wave-lengths at the mean operating frequency is made of difierent cross section, such that the characteristic impedance of the 11/4 section is equal to the square root of the product of the impedance of the waveguide (portion 52b of line 52) and the impedance of the traveling wave tube 12. This will be recognized as the well-known quarter-wave transformer.

In the foregoing discussion, the ramps have been described as attached to the waveguide. It is possible, of course, to make the waveguide of such configuration that ridges are provided by the contour of the waveguide walls, as shown in FIG. 11. In this case, the conductors 54 and 55 of the transmission line 52 are connected directly to the reentrant portions 36' and 36 of the waveguide 36. The same construction would apply, of course to the input transmission line 32.

This invention is not limited to the particular details of construction, materials and processes described, as

many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. In combination, a traveling wave tube including a slow wave signal propagating structure having at least two terminals, a waveguide for transmitting electromagnetic energy, said waveguide including two portions each having ridges extending toward one another from opposite parts of said waveguide and forming the boundary of a space between which an electric field may be maintained, and a pair of two conductor transmission lines each having one end coupled to a different one of said ridged portions of said waveguide and the other end electrically coupled to one of said terminals, said conductors of said transmission lines including a portion of reduced size whose length is any odd number of quarter wave lengths long at the mean operating frequency of said tube.

2. In combination, a traveling wave tube including a slow wave signal propagating structure having input and output terminals, a waveguide for transmitting electromagnetic ener y having collinear input and output sections separated from each other by a radio frequency barrier, each of said sections of said waveguide including a ridged portion extending toward said barrier from opposite parts of said waveguide and forming the boundary of a space between which an electric field may be maintained, a pair of two conductor transmission lines one end of each line coupled to one of said ridged portions and the other end of each line electrically coupled to a terminal of said slow wave structure.

3. In combination, a traveling wave tube including a slow wave signal propagating structure having input and output terminals, a waveguide for transmitting electromagnetic energy having input and output sections separated from each other by a radio frequency barrier, each of said sections of said waveguide including a ridged portion extending toward said barrier from opposite parts of said waveguide and forming the boundary of a space between which are a pair of two conductor transmission lines one conductor of each line having one end coupled to one of said ridged portions and the other end of each line electrically coupled to a terminal of said slow wave structure, the distance between the conductors of each of said transmission lines being progressively varied along the Iength of said line.

4. In combination, a traveling wave tube including a slow wave signal propagating structure having input and output terminals, a waveguide for transmitting electromagnetic energy having input and output sections separated from each other by a radio frequency barrier, each of said sections of said waveguide including a rigid portion extending toward said barrier from opposite parts of said waveguide and a pair of two conductor transmission lines, one conductor of each line having one end coupled to one of said rigid portions and the other end of each line electrically coupled to a terminal of said slow wave structure, said conductors of each of said transmission lines including a portion of reduced size Whose length is any odd number of quarter wave lengths long at the mean operatingfrequency of said tube.

5. In combination, a traveling wave tube including a slow wave signal propagating structure having input and output terminals, a waveguide for transmitting electromagntic energy having collinear input and output sections separated from each other by a radio frequency barrier, each of said sections of said waveguide including at least two ridges extending toward one another from opposite parts of said waveguide and forming the boundary of a space between which an electric field may be maintained, said ridges being tapered over a region thereof to provide a space therebetween of progressively changing dimensions, a pair of two conductor transmission lines, one conductor of each line having one end coupled to a different one of said ridges of said input section and the other end of each line electrically coupled to a terminal of said slow wave structure.

6. In combination, a traveling wave tube including a slow wave signal propagating structure having input and output terminals, a waveguide for transmitting electromagnetic energy having collinear input and output sections separated by means of a radio frequency barrier arranged transverse to the longitudinal axis of said waveguide, each of said sections of said waveguide including at least two ridges extending toward one another from opposite walls of said waveguide and forming the boundary of a space between which an electric field may be maintained, a pair of two conductor transmission lines, one conductor of each line having one end coupled to one of said ridges of a diiferent one of said sections and the other end of each line electrically coupled .to a terminal of said slow wave structure, said conductors extending parallel to and adjacent to said barrier.

7. In combination, a traveling wave tube including a slow wave signal propagating structure having input and output terminals, said structure being contained within an evacuated envelope, a waveguide for transmitting electromagnetic energy having collinear input and output sections separated from each other by means of a radio frequency barrier arranged transverse to the longitudinal axis of said waveguide and extending within said envelope, each of said sections of said waveguide including two ridges extending toward one another from opposite walls of said waveguide and forming the boundary of a space between which an electric field may be maintained, a pair of two conductor transmission lines, each conductor of each line having one end coupled to a diiferent one of said ridges and the other end of each line electrically coupled to a terminal of said slow wave structure, said conductors extending parallel to and adjacent to said barrier.

8. In combination, a traveling wave tube including a slow wave signal propagating structure having input and output terminals, said structure being contained within an evacuated envelope, a waveguide for transmitting electromagnetic energy having collinear input and output sections separated from each other by means of a radio frequency barrier arranged transverse to the longitudinal axis of said waveguide and extending within said envelope, each of said sections of said waveguide including two ridges extending toward one another from opposite walls of said waveguide and forming the boundary of a space between which an electric field may be maintained, a pair of two conductor transmission lines, one conductor of each line having one end coupled to a dilferent one of said ridges and the other end of each line electrically coupled to a terminal of said slow wave structure, said conductors extending parallel to and adjacent to said barrier, said conductors of said transmission lines including a portion of reduced size whose length is any odd number of quarter wave lengths at the mean operating frequency of said tube.

9. In combination, a traveling wave tube including a slow wave signal propagating structure having input and output terminals, a waveguide for transmitting electromagnetic energy having collinear input and output sections separated from each other by means of a conductive diaphragm, each of said sections of said waveguide including at least two ridges extending toward one another from opposite walls of said waveguide and forming the boundary of a space between which an electric field may be maintained, a first parallel conductor transmission line including two conductors each of which have one end coupled to a different one of said ridges of said input section and the other end electrically coupled to one of the input terminals of said slow wave structure, a second parallel conductor transmission line including two conductors each of which have one end electrically coupled to a different one of said ridges of said output section and the other end electrically coupled to one of the output terminals of said slow wave structure.

References Cited by the Examiner UNITED STATES PATENTS 2,317,503 4/ 1943 Usselrnan 333- 2,633,493 3/1953 Cohn 333-34 2,691,731 10/1954 Miller 33334 2,786,959 3/1957 Warneke et al. 315-393 2,807,784 9/1957 Lerbs 333-34 2,922,961 1/ 1960 Robertson 33334 3,113,239 12/1963 Hass 315-3977 FOREIGN PATENTS 1,279,813 11/ 1961 France.

HERMAN KARL SAALBACH, Primary Examiner.

NORMAN H. EVANS, Examiner.

CHESTER L. JUSTUS, D. G. BREKES, R. A. FARLEY,

M. R. WILBUR, Assistant Examiners. 

1. IN COMBINATION, A TRAVELING WAVE TUBE INCLUDING A SLOW WAVE SIGNAL PROPAGATING STRUCTURE HAVING AT LEAST TWO TERMINALS, A WAVEGUIDE FOR TRANSMITTING ELECTROMAGNETIC ENERGY, SAID WAVEGUIDE INCLUDING TWO PORTIONS EACH HAVING RIDGES EXTENDING TOWARD ONE ANOTHER FROM OPPOSITE PARTS OF SAID WAVEGUIDE AND FORMING THE BOUNDARY OF A SPACE BETWEEN WHICH AN ELECTRIC FIELD MAY BE MAINTAINED, AND A PAIR OF TWO CONDUCTOR TRANSMISSION LINES EACH HAVING ONE END COUPLED TO A DIFFERENT ONE OF SAID RIDGED PORTIONS OF SAID WAVEGUIDE AND THE OTHER END ELEC- 